Fuel cells have been proposed as a clean, efficient and environmentally friendly source of power which can be utilized for various applications. A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode, i.e. anode where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations. The electrons are conducted from the anode to a second electrode, i.e. cathode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the cathode. Simultaneously, an oxidant, such as oxygen gas or air is introduced to the cathode where the oxidant reacts electrochemically in the presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The anode may alternatively be referred to as a fuel or oxidizing electrode, and the cathode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:H2→2H++2e−½O2+2H++2e−→H2O 
The external electrical circuit withdraws electrical current and thus receives electrical power from the fuel cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction. Accordingly, the use of fuel cells in power generation offers potential environmental benefits compared with power generation from combustion of fossil fuels or by nuclear activity. Some examples of applications are distributed residential power generation and automotive power systems to reduce emission levels.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a separate cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation. Piping and various instruments are externally connected to the fuel cell stack for supplying and controlling the fluid streams in the system. The stack, housing, and associated hardware make up the fuel cell unit.
There are various known types of fuel cells. For example, proton exchange membrane (PEM) fuel cells are one of the most promising replacements for traditional power generation systems, as a PEM fuel cell enables a simple, compact fuel cell to be designed, which is robust and which can be operated at temperatures not too different from ambient temperatures. Usually, PEM fuel cells are fuelled by pure hydrogen gas, as it is electrochemically reactive and the by-products of the reaction are water and heat, which is environmentally friendly. A conventional PEM fuel cell usually comprises two flow field plates (bipolar plates), namely, an anode flow field plate and a cathode flow field plate, with a membrane electrode assembly (MEA) disposed therebetween. The MEA includes the actual proton exchange membrane and layers of catalyst for fuel cell reaction coated onto the membrane. Additionally, a gas diffusion media (GDM) or gas diffusion layer (GDL) is provided between each flow field plate and the PEM. The GDM or GDL facilitates the diffusion of the reactant gas, either the fuel or oxidant, to the catalyst surface of the MEA while providing electrical conductivity between each flow field plate and the PEM.
Each flow field plate typically has three apertures or openings at each end, each aperture representing either an inlet or outlet for one of fuel, oxidant and coolant. However, it is possible to have multiple inlets and outlets on flow field plates for each reactant gas or coolant, depending on the fuel cell or stack design. When a fuel cell is assembled, the anode flow field plate of one cell abuts against the cathode flow field plate of an adjacent cell. These apertures extend throughout the thickness of the flow field plates and align to form elongate distribution channels extending perpendicular to the flow field plates and through the entire fuel cell stack when the flow field plates stack together to form a complete fuel cell stack. A flow field usually comprises at least one, and in most cases, a plurality of open-faced flow channels that fluidly communicate between an appropriate inlet and outlet. As a reactant gas flows through the channels, it diffuses through the GDM and reacts on the MEA in the presence of the catalyst. A continuous flow through ensures that, while most of the fuel or oxidant is consumed, any contaminants are continually flushed through the fuel cell. The flow field may be provided on either face or both faces of the flow field plate. Typically, fuel or oxidant flow fields are formed respectively on the face of the anode and cathode flow field plate that faces toward the MEA (hereinafter; referred to as the “front face”). A coolant flow field may be provided on either the face of either of the anode or cathode flow field plate that faces away from the MEA (hereinafter, referred to as the “rear face”).
When a complete fuel cell stack is formed, a pair of current collector plates are provided immediately adjacent the outmost flow field plates to collect current from the fuel cell stack and supply the current to an external electrical circuit. A pair of insulator plates are provided immediately outside of the current collector plates and a pair of end plates are located immediately adjacent the insulators. A seal is provided between each pair of adjacent plates. The seal is usually in the form of gaskets made of resilient materials that are compatible with the fuel cell environment. A fuel cell stack, after assembly, is commonly clamped to secure the elements and ensure that adequate compression is applied to the seals and active areas of the fuel cell stack. This method ensures that the contact resistance is minimized and the electrical resistance of the cells is at a minimum.
Various designs of the flow field have been known. One commonly known flow field pattern can be found in U.S. Pat. No. 4,988,583. A single continuous open-faced fluid flow channel is provided on one surface of a flow field plate. The flow channel has an inlet and an outlet, respectively located near two opposite ends of the flow field plate. The inlet and outlet are in fluid communication with gas distribution manifolds in the fuel cell stack. The flow channel traverses the surface of the flow field plate in a plurality of passes. The flow channel in the serpentine form provides a long flow channel without increasing the dimension of the flow field plate, thereby allowing somewhat sufficient diffusion of reactant gases from the flow channel to the MEA.
Extensive improvements have been made on the basis of this “serpentine” flow channel concept. These improvements can be found in U.S. Pat. Nos. 6,099,984 and 6,309,773. However, these designs suffer from a number of problems. Serpentine flow channels cause greater pressure drop when the reactant gases flow across the flow field. This is a serious problem that significantly affects the performance of the fuel cell when the fuel cell is operating under a relatively low pressure, for example, ambient pressure. The gas distribution in these designs is also not uniform along the tortuous flow paths. The gas flow is more turbulent in the serpentine flow field, making it more difficult to control the flow, pressure or temperature of the reactant gases. In addition, tortuous flow paths provide more places for water or contaminants to accumulate in the channels, increasing the risk of flooding or poisoning the fuel cell.
Another problem associated with most of flow field designs is the ribs and channels on the anode flow field plate often offset with those on the cathode flow field plates when placed in a fuel cell stack. As mentioned above, the anode and cathode flow field plate are placed adjacent the opposite side of the MEA and reactant gases flow through the chambers formed by GDM and the open-faced channels in the flow field. Since pressure is often applied on a fuel cell stack, the MEA and GDM are thus subject to shearing force, which may eventually damage the MEA. The offset of the ribs also impedes the distribution of reactant gases across GDM, reducing the fuel cell efficiency.
It can be appreciated from the previous discussion that a further problem in conventional fuel cell is that the sealing is often complicated. Various apertures on the MEA, flow field plates, current collector plates, etc must be sealed. In addition, as mentioned, a seal is required between each pair of adjacent plates and each seal would be of complex and elaborate construction. For any one reactant gas, it is conceivable to provide a seal that completely encloses all of the flow field and its inlet and outlet on the corresponding, first flow field plate. This will enable a good seal to be formed between that flow field plate and the MEA. However, on the other side of the MEA, it is necessary to provide a seal that completely encloses an aperture on a second flow field plate that corresponds to inlet and outlet on the first flow field plate. In this configuration, part of the membrane would lie over open channels on the first flow field plate, and hence not be properly supported, thereby running the risk of there being inadequate sealing, resulting in a mixing of gases, which is highly undesirable.
Therefore, there remains a need for a fuel cell flow field plate that provides a small pressure drop across the fluid flow field and more uniform gas distribution. Preferably, the flow field plate reduces the shearing effects on the MEA and simplifies sealing between flow field plates.